This invention relates to a biaxial tensile stage and more particularly to such a stage for fabrication and tuning of micro and nano scale wrinkle patterns that are generated upon compression of thin films.
Wrinkling is a strain-driven self-organization phenomenon that is commonly observed in natural systems over a wide length scale. Recently, this phenomenon has been incorporated into engineered systems to generate micro and nano scale patterns. For example, wrinkling of bilayer materials has been used to fabricate periodic sinusoidal patterns for thin film metrology, stretchable electronics, and microfluidics applications. Due to its inherent affordability and manufacturing scalability, pattern generation via wrinkling is an attractive potential alternative to more expensive cleanroom-based techniques such as e-beam lithography. However, practical import of this process is limited by the lack of flexibility, i.e., due to the inability to fabricate a variety of complex 2-D patterns. This is due to the limited ability of current tensile stages to provide the means to apply large, accurate, and/or non-equibiaxial strains within a small footprint. It is the goal of this invention to provide such a stage thereby enabling one to explore and access a wider design space for fabricating and tuning wrinkled patterns.
Wrinkles in compressed bilayer systems are formed due to buckling-based instabilities. The mechanism of wrinkling is similar to Euler buckling of columns under compression. A schematic of this process is illustrated in FIG. 1. Essential elements of these bilayer systems are: (i) a film 10 that is thin relative to the base, (ii) mismatch in the elastic moduli of the film and the base 12 with the film being stiffer than the base, and (iii) loading conditions 14 that generate in-plane compressive strain in the film. In such systems, the state of pure compression becomes unstable beyond a critical strain and wrinkles are formed via periodic bending of the film/base. The period of wrinkles (λ) is determined by the competing dependence of strain energy on period in the film versus in the base. The amplitude (A) is determined by the amount of applied compressive strain. Several different techniques have been developed in the past to (i) generate and join/bond the film to the base, (ii) generate moduli mismatch, and (iii) apply uniaxial and equibiaxial strains to the film. Analytical and computational predictive models for uniaxial and equibiaxial strains have also been developed. As such, these techniques and models provide a framework for performing predictive design and fabrication of periodic wrinkle patterns.
Although current techniques and models are a valuable toolkit for predictive design and fabrication of wrinkled patterns, they are still inadequate in satisfying the need for a variety of different complex patterns. This is primarily because only a small subset of the feasible design space is accessible via existing experimental techniques. The set of wrinkle patterns that can be fabricated is limited by the achievable range and types of compressive strains. For example, (i) below a threshold strain, only the single-period sinusoidal mode can be achieved via uniaxial strains and (ii) only a limited set of symmetric 2-D modes can be achieved via equibiaxial strains. Existing techniques that rely on thermal expansion or volumetric swelling to generate strains can provide only a limited set of strain states. For example, mismatched thermal expansion of an isotropic film on an isotropic base generates equibiaxial strains. Due to this, exploring the design space for large uniaxial or non-equibiaxial strains becomes a material selection problem. This coupling between strain and materials can be eliminated by using mechanical stages to introduce strains via stretching of the base layer. However, existing biaxial mechanical stages are often too large to use within vacuum chamber based equipment that are necessary for generation of thin films during wrinkle fabrication. Thus, there is a need to develop a mechanical stage that has a small form factor and provides the means to apply large, accurate, and non-equibiaxial strains.
Compact mechanical stages that are capable of providing large non-equibiaxial strains become a necessity when a variety of complex wrinkle patterns are required. The present biaxial tensile stage is compact and is capable of providing uniaxial and sequential non-equibiaxial stretching. The stage also has alignment features that enable one to register it to a vision system. This enables performing real-time in-situ visualization of the wrinkles as stretches are varied. Thus, this system is (i) an effective tool to experimentally study and characterize wrinkle formation and (ii) manufacturing equipment for low-cost fabrication of micro and nano scale patterns. By enabling fabrication of complex micro and nano scale patterns, this system reduces the overall cost of manufacturing micro and nano-enabled products by a factor of at least 10.